The present invention relates to a seat belt system that is provided on a vehicle, for example, an automobile, for the purpose of protecting an occupant. More particularly, the present invention relates to a seat belt retractor designed so that a reel shaft for winding up a webbing is locked from rotating when an emergency situation occurs, thereby preventing the webbing from being pulled out.
A typical conventional seat belt retractor mounted on a vehicle, for example, an automobile, includes a reel shaft for winding up a webbing, a frame rotatably supporting both ends of the reel shaft, lock means disposed in between the frame and the reel shaft and arranged to allow rotation of the reel shaft under normal circumstances and to be activated, when need arises, so as to prevent rotation of the reel shaft in at least a direction in which the webbing is unwound, deceleration sensing means operating when deceleration exceeding a predetermined value acts on the vehicle, and means for activating the lock means in response to the operation of the deceleration sensing means, as disclosed, for example, in the specification and drawings of U.S. Pat. No. 4,796,918.
In such a seat belt retractor, the reel shaft is freely rotatable under normal circumstances. Therefore, the webbing can be unwound freely, and the occupant can move forwardly as desired. When an emergency situation occurs, e.g., when a high degree of deceleration acts on the vehicle, the deceleration sensing means operates on sensing the deceleration, causing the lock activating means to activate the lock means so as to lock the reel shaft from rotating. Consequently, the webbing is prevented from being pulled out, and the occupant's body is restrained by the webbing. Thus, inertial forward movement of the occupant's body is prevented.
Incidentally, in many of conventional seat belt retractors of the type described above, the lock activating means has a lock means activating member that rotates together with the reel shaft under normal circumstances and that activates the lock means by relative rotation caused between the same and the reel shaft by the operation of the deceleration sensing means. The lock activating means further has webbing pull-out speed sensing means (webbing sensor) operating when the webbing is unwound at a pull-out speed exceeding a predetermined value. The webbing pull-out speed sensing means has an inertia member provided on the lock means activating member so as to be pivotable between an inoperative position where it allows rotation of the lock means activating member, and an operative position where it prevents rotation of the lock means activating member in at least the direction in which the webbing is unwound,
With this type of retractor, when the webbing is unwound rapidly at a pull-out speed higher than a predetermined value, the webbing pull-out speed sensing means senses the excessive webbing pull-out speed and locks the lock means activating member from rotating so as to cause relative rotation between the lock means activating member and the reel shaft, thereby activating the lock means to lock the reel shaft from rotating, and thus preventing unwinding of the webbing.
In this type of seat belt retractor, however, since an inertia member is pivotably attached to the lock means activating member, when the lock means activating member rotates as the webbing is unwound under normal circumstances, the inertia member also revolves around the axis of rotation of the lock means activating member. The revolution of the inertia member causes centrifugal force to act on the inertia member. Therefore, the inertia member may pivot about the axis of rotation thereof so as to lock the lock means activating member from rotating. Thus, the conventional seat belt retractor suffers from the problem that the stability of the webbing sensor is not always satisfactory.